1. Field of the Invention
The present invention generally relates to a nozzle system for a downhole drill bit. More particularly, the present invention relates to an improved drill bit and nozzle system which better manages fluid flow and decreases erosion of the drill bit body and methods for the design and operation of downhole drill bits to reduce erosion.
2. Description of the Prior Art
Subterranean drill bits are used in a number of different applications and under a variety of environments. In this connection, subterranean drill bits are conventionally used in mining, construction, oil and gas exploration, and oil and gas production.
There are two general types of commercially available drill bits. A roller bit utilizes steel teeth or tungsten carbide inserts. A fixed cutter bit describes a drill bit that does not employ a moving cutting structure. Fixed cutter bits include polycrystalline diamond compact (PDC), thermally stable polycrystalline (TSP), natural diamond and other bits which do not use a diamond material as a cutting element.
A conventional downhole drill bit includes a shank with a threaded connection for mating with a drilling motor or a drill string. The shank can include a pair of wrench flats, sometimes referred to as "breaker slots", to apply the appropriate torque to make-up the thread shank. The distal or bottom end of the drill bit contains the cutting structure, be it roller or fixed cutter as described above. The bit body further includes a central bore which allows fluid communication between the borehole and the drill string. This central bore terminates in several fluid openings disposed about the bit face and adapted to circulate pressurized fluid over the cutting surface. These openings are provided with nozzles which control fluid flow therethrough.
By utilizing a small nozzle orifice in the nozzle body, fluid velocity through the central bore is increased with a proportionate increase in the pressure required to pump a given volume of fluid. Conversely, by increasing the nozzle orifice, the pressure to pump a given volume of fluid decreases and the fluid velocity decreases. By selecting the nozzle orifice size, the operator is able to control the velocity and pressure of the fluid flow through the bit.
Internal erosion in and around nozzle bodies is a major problem in the longitivity of the bit and, indirectly, the economy of the drilling operation. Drilling fluid generally contains a percentage of entrained solids, many of which are highly abrasive. Given the presence of such entrained solids, increased fluid velocity generally results in a proportionate increase in the erosion of the bit body. Consequences of such erosion include eroded areas through the bit structures which result in a loss of hydraulic pressure and necessitates a trip out of the bore hole to replace a bit damaged as a result of fluid erosion.
There are a number of disadvantages associated with traditional subterranean drill bits and methods for their operation. One such disadvantage relates to internal erosion of the drill bit body caused during operation of the bit by fluid circulation. Factors contributing to such erosion include mud weight, mud viscosity and flow velocity through the drill bit.
A second disadvantage is internal erosion of the drill bit body caused by geometrical discontinuities in the fluid openings leading to the nozzle bodies. In this connection, sharp angles create fluid flow separation and high shear layer stresses as well as adding to the erosive capabilities of the fluid.
Other disadvantages lie in the method of operation of traditional drill bits. Conventional methods of operation given known design parameters fail to maintain laminar fluid flow within the bit body during operation, thereby resulting in enhanced erosion.